At present, various types of memory devices are used as data storage devices in various electronic equipments. Among such memory devices, there are memory devices, such as a flash memory device, an EPROM (Erasable Programmable ReadOnly Memory) device, an EEPROM (Electrically Erasable Programmable Read-Only Memory) device, and the like into which binary data is stored or set by storing electric charges into a storage cell or cells and from which the binary data is freely retrievable.
Each of these memory devices generally comprises a number of memory cells as a plurality of data storage cells. The memory cells are disposed in a two dimensional array or in a matrix. Each of these memory cells is constituted of a field effect transistor circuit comprising a floating gate (FG) and a tunnel oxide film, and stores electric charges injected into the floating gate to store data.
In such memory devices, a plurality of word lines and a plurality of bit lines are disposed in vertical and lateral directions, and coupled with the memory cells disposed in the two dimensional array. Also, as a charge injection means, driver circuits and control circuits are coupled to these word lines and bit lines.
In the data storage device having the above-mentioned structure, it is possible to freely store digital data and to freely read out or retrieve the stored digital data. In this case, a series of binary data bits constituting the digital data are allocated to a series of memory cells, and electric charges are injected into each of the memory cells according to the digital data. Each of the memory cells stores electric charges selectively injected thereinto and therefore a series of binary data bits are written into the data storage device, which binary data bits can be freely read out from the data storage device.
In some of the above-mentioned data storage devices, a data verify operation is performed to compensate for incomplete injection of electric charges. In the data storage device performing the data verify operation, a threshold voltage of each memory cell is measured immediately after injecting electric charges into each memory cell, and it is judged if the electric charges are not appropriately injected into each memory cell. Then, electric charges are injected again into a memory cell or cells into which it is determined that electric charges are not stored properly. Thereby, incomplete or defective injection of electric charges is compensated, and it is possible to reliably write digital data into a data storage device.
However, even if electric charges are injected into a floating gate of a memory cell and thereby stored in the memory cell, the electric charges are sometimes decreased due to thermal emission or current leakage as time elapses. If the electric charges stored in a memory cell are decreased, it becomes difficult to correctly read out digital data written into and stored in the memory cell.
In order to solve such problem, a data storage device is disclosed in Japanese patent laid-open publication No. 9-306182. In the data storage device disclosed in this publication, multi-valued data is written into and stored in each of non-volatile memory cells. Quantity of electric charges in each memory cell is measured regularly, and if a result of the measurement indicates that the quantity of electric charges is out of an allowable range, data write or data setting operation is again performed.
In the data storage device disclosed in the above-mentioned Japanese publication, since the cause of secular loss of electric charges is considered to be thermal emission and current leakage, and, as shown in FIG. 7, rate of leakage of a current is assumed to be approximately constant. Therefore, even when charge storage ability of a memory cell is deteriorated by the repeated application of stress and the like, it is expected that necessary electric charges can be maintained by regularly repeating data setting or writing to the memory cell, as shown in FIG. 8.
However, in a recent data storage device, precision in manufacturing and various property of materials are improved, and decrease of electric charges due to thermal emission and current leakage is almost resolved. Therefore, it is hardly necessary to measure a threshold voltage of a memory cell regularly and to inject electric charges again as mentioned in the above-mentioned Japanese publication. Also, occurrence of inferior charge injection is very rare at present, and the above-mentioned verify operation is also omitted in general products.
For example, in a recent data storage device such as a flash memory device, an EPROM device, an EEPROM device and the like, loss of electric charges due to thermal emission and current leakage is almost resolved. Therefore, it is not necessary to measure a threshold voltage of a memory cell regularly and to inject electric charges again frequently. Also, occurrence of inferior charge injection is almost resolved, and the above-mentioned verify operation can also omitted.
However, even in a recent data storage device, when data write and/or data erasure are repeatedly performed, there is a possibility that property or ability of storing electric charges in a memory cell is sometimes deteriorated. The main cause of such deterioration is considered to be as follows. When stress such as movement of electric charges and the like is repeatedly applied to a tunnel oxide film, trap sites are produced in the tunnel oxide film, and electric charges stored in a floating gate move to the trap sites, thereby electric charges stored in the floating gate is decreased.
It may be possible to resolve such reduction of electric charges in the floating gate caused by the trap sites of the tunnel oxide film, by a general verify operation or the rewrite operation disclosed in the above-mentioned Japanese publication. However, since the verify operation is performed immediately after injection of electric charges, there is a strong possibility that the verify operation is performed before the movement of electric charges from the floating gate to the trap sites in the tunnel oxide film. It is considered that electric charges move after the verify operation.
Also, in the data storage device described in the above-mentioned Japanese publication, it is assumed that a threshold voltage of each memory cell drops at a predetermined rate due to thermal emission and current leakage. Thus, the threshold voltage of each memory cell is regularly measured and injection of electric charges is repeatedly performed. Therefore, it is difficult to efficiently and effectively compensate for loss of stored electric charges caused by the trap sites.
Especially, in case the data storage device is integrated into an electronic equipment such as a computer system and the like, it is easily possible to regularly detect a threshold voltage of a memory cell and to repeat injection of electric charges. However, in case the data storage device is constituted as a memory card and the like, it is difficult to perform such operation because of power consumption and the like.